The invention relates to a method of displaying characters with a border on a display device as a raster consisting of raster lines and raster columns, the character information being derived from a memory, as well as to a device for displaying characters with a border on a raster consisting of raster lines and raster columns, and comprising a data source for supplying the character information.
A method of this kind is known from U.S. Pat. No. 5,003,303. Therein, the character information for each raster point of the character contains a multidigit value. This value is transformed into a part of the overall value range, and in parallel therewith an expanded form of the character element is formed in the other part of the value range, for example in that for each raster field it is tested whether a character element is present within a matrix field of given size around the relevant raster field. After rotation or shifting of the character, if applicable, the two value ranges are separately backtransformed into the overall value range again, and a selectable colour is assigned to the border information. Finally, the overall character information with border is stored in an image memory wherefrom display takes place.
The image memory is necessary because the many processing steps required for each individual raster point of the character to be displayed cannot be executed at the display speed. Such an image memory, however, represents a substantial expenditure.
In order to save such a character memory, the image information of the characters to be displayed, for example letters or digits character-wise stored in a character memory and addressed by character codes, is read often in synchronism with the display on the display device, i.e. in synchronism with the deflection of the electron beam in the case of a picture tube. The characters are customarily reproduced in a selectable colour on a background which is also selectable. The background may also be transparent, for example when the characters are to be superposed on a television image in given positions. In case the colour of the image part on which the characters are to be superposed happens to be the same as the colour selected for the characters and the background of the characters is transparent, the characters displayed cannot be recognized.
One possibility of making the characters recognizable still on a background of arbitrary colour consists in providing the characters with a border on at least some sides; such a border is also referred to as a shade. This border is reproduced in a different colour, so that either the characters or the border can always be reliably recognized, regardless of the colour of the background. For other reasons, for example so as to achieve a more aesthetic display, it may also be desirable to display characters with a border. This border may be desired at one or two sides, for example at the top and at the right, so that it appears as a shade; however, it may also be desirable to provide a border at three or at all four sides.
Therefore, it is an object of the invention to provide a method of the kind set forth which enables the display of arbitrary characters with a minimum expenditure, notably without using an image memory, and with a border at arbitrarily selectable sides.